Isotopic composition of dissolved inorganic nitrogen in high mountain lakes: variation with altitude in the Pyrenees

Abstract

Abstract. Nitrogen deposition in remote areas has increased, but the effect on ecosystems is still poorly understood. For aquatic systems, knowledge of the main processes driving the observed variation is limited, as is knowledge of how changes in nitrogen supply affect lake biogeochemical and food web processes. Differences in dissolved inorganic nitrogen (DIN) between lakes cannot be understood without considering catchment characteristics. In mountains, catchment features (e.g., thermal conditions, land cover) vary considerably with elevation. The isotopic composition of nitrogen (δ 15 N) is increasingly used to study aquatic ecosystem dynamics. Here we explore the variability of δ 15 N in DIN in high mountain lakes and show that environmental conditions that change with altitude can affect the isotopic ratio.We measured ammonium and nitrate δ 15 N values in atmospheric deposition, epilimnetic water, deep chlorophyll maximum water (DCMW) and sediment pore water (SPW) from eight mountain lakes in the Pyrenees, both above and below the treeline. Lakes showed relatively uniform δ 15 N-NH + 4 values in SPW (2.2±1.6‰), with no variation corresponding to catchment or lake characteristics. We suggest that organic matter diagenesis under similar sediment conditions is responsible for the low variation between the lakes.In the water column, the range of δ 15 N values was larger for ammonium (−9.4‰ to 7.4‰) than for nitrate (−11.4‰ to −3.4‰), as a result of higher variation both between and within lakes (epilimnetic vs. DCM water). ForCorrespondence to: M. Bartrons (mbartrons@ceab.csic.es) both compounds part of the difference correlated with altitude or catchment features (e.g., scree proportion). Based on concentration, chemical and isotopic tendencies, we suggest that patterns arise from the distinct relative contributions of two types of water flow paths to the lakes: one from snowpack melting, with little soil interaction; and another highly influenced by soil conditions. The snow-type flow path contributes low DIN concentrations depleted in 15 N, whereas the soil-type flow path contributes high nitrate concentrations with higher δ 15 N. The proportion of these two types of source correlates with average catchment features when there is extensive snow cover during spring and early summer and probably becomes more dependent on local characteristics around the lake as summer advances. Lake depth and pore water ammonium concentrations, among other features, introduce secondary variation. In the context of nitrogen deposition studies, lakes with higher snow-type influence will probably register changes in N deposition and pollution sources better, whereas lakes with higher soil-type influence may reflect long-term effects of vegetation and soil dynamics.

Catchment‐mediated atmospheric nitrogen deposition drives ecological change in two alpine lakes in SE Tibet

“…4b, f). Bartrons et al (2010) have suggested that vegetated soils can contribute to high d 15 N, and the difference in catchment characteristics and morphologies between the two lakes here (Fig. Catchment soils are the largest storage pool for N, although nutrient uptake by terrestrial vegetation is also important (Fenn et al, 1998).…”

Semi-annual carbon and nitrogen isotope variations in the water column of Lake Bled, NW Slovenia

“…The concentrations of NH + 4 were not sufficient for isotope measurements in the water column, except at the bottom where we found that δ 15 N values of NH + 4 averaged +3.8 ± 0.1 ‰. δ 15 N NH 4 values in the water column found in high mountain lakes in the Pyrenees ranged from −9.4 to +7.4 ‰, depending on snow-pack melting or soil conditions (Bartrons et al, 2010). On the other hand, the 2 ± 1.6 ‰ in sediment pore waters were found to be uniform, with no variations corresponding to catchment or lake characteristics, and similar to values found in our study at the bottom of the water column.…”

“…Atmospheric deposition is the dominant source of N compounds in most high mountain and oligotrophic lakes (Ostrom et al, 1997;Wookey et al, 2009;Vreča and Muri, 2010). In addition, in high mountain lakes, another N source can be snow-pack melting influenced by soil conditions (Bartrons et al, 2010). More anthropogenic nitrate loading derived from fertilizers, manure and/or sewage can be observed in lakes located in urban areas (Townsend-Small et al, 2009).…”

Regional variability in the atmospheric nitrogen deposition signal and its transfer to the sediment record in Greenland lakes

“…This increases the local supply of N that occurs during spring thaw, resulting in a temporal mismatch between availability and biological uptake, thereby enhancing availability for leaching into surface waters during snow melt. However, Bartrons et al () showed how soil type in a catchment contributes to higher δ 15 N than that related to snow melt processes, which may reflect local soil dynamics and vegetation effects. Increased decomposition rates associated with warming and/or vegetation changes are also reflected in higher δ 15 N in soil organic matter and consequently in leached from soils to lakes, which may be recorded in lake sediments.…”